A printed wiring board, printed circuit or the like has sites for accepting integrated circuits, transistors and other circuit components. Surrounding each site is an array of electrically conductive bonding pads to which the leads of the component suited to that site are electrically and mechanically connected. Conductor runs connect the various pads to each other and to terminal pads often located at an edge of the printed circuit to connect the circuit to the outside world.
The circuit components often have flat wire leads which extend out from the sides of the component, are bent down more or less 90.degree., and terminate in a short horizontal foot. In other words, each lead is shaped more or less like the letter L. The positions and dimensions of these leads correspond to the positions and dimensions of the bonding pads around the printed circuit site for that component such that the feet of these leads fit flush on and substantially cover the bonding pads.
FIG. 2 shows a typical component 10 whose lead 12 terminates in a horizontal foot 12a which rests on a bonding pad 14 of a printed circuit 16. The bonding pad 14 consists of a layer 14a of conductive metal, e.g. copper, pre-coated with a thin layer 14b of solder. Conventionally, to bond the lead to the pad, the horizontal lead foot l2a is irradiated by a vertical laser beam L which heats the lead foot sufficiently to remelt the solder and the molten solder transfers the thermal energy to the pad 14. As soon as the laser beam L is removed, the solder solidifies thereby connecting the lead 12 to the pad 14 both electrically and mechanically.
This prior bonding procedure has certain disadvantages. If thermal contact between the lead and the pad is limited as would be the case if the pad is domed, for example, the lead may not be coplanar with or in good thermal contact with the solder pad. In this event, the laser's beam energy may overheat the lead and destroy the inner lead bond or otherwise damage the component before the solder melts.
Also, during the reflow soldering process, the electrical component and its leads are positioned with respect to the printed circuit by a jig or fixture. Since the laser beam is conventionally aimed at the foot of the lead during bonding as shown in FIG. 2, that part of the lead cannot be obscured by hold-down appliances on other parts of the fixture. To maximize the available real estate for these appliances, the parts of the fixture overlying each lead can be made transparent to the laser beam. However, this complicates, and increases the cost of, the fixture.
It should be mentioned also that some of the electrical components bonded to substrates, circuit boards and the like have J-shaped leads. In other words, the end or foot of each lead is curled under the component. If the prior laser bonding process is used to connect components of this type to a printed circuit, the laser must be repositioned so that it delivers the laser beam at an angle to the normal direction against the side of the lead. This requires that the laser be supported by a movable arm or other mount that allows such articulation.